1. Field of the Invention
This invention relates to electrical circuit breakers and more particularly to electrical circuit breakers which incorporate apparatus which restricts the travel of the circuit breaker operating handle when the contacts of the circuit breaker are closed due to an obstruction or contact welding.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300% of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation the contacts may be opened by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, the magnetic repulsion member is connected between a pivotally mounted contact arm and a stationary conductor. The magnetic repulsion member is a generally V-shaped member defining two legs. During high level short circuit conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of the current flowing therethrough which, in turn, causes the pivotally mounted contact arm to open.
In a multipole circuit breaker, such as a three-pole circuit breaker, three separate contact assemblies having magnetic repulsion members are provided; one for each pole. The contact arm assemblies are operated independently by the magnetic repulsion members. For example, for a high level short circuit on the A phase, only the A phase contacts would be blown open by its respective magnetic repulsion member. The magnetic repulsion members for the B and C phases would be unaffected by the operation of the A phase contact assembly. The circuit breaker operating mechanism is used to trip the other two poles in such a situation. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and feed the fault.
In the other automatic mode of operation, the contact assemblies for all three poles are tripped together by a current sensing circuit and a mechanical operating mechanism. More particularly, current transformers are provided within the circuit breaker housing to sense overcurrent conditions. When an overcurrent condition is sensed, the current transformers provide a signal to electronic circuitry which actuates the operating mechanism to cause the contacts to be separated.
The operating mechanism of the circuit breaker is designed to rapidly open and close the separable contacts thereby preventing the moveable contact from stopping at any position which is intermediate the fully open or fully closed position. This accomplishes two purposes. First, when the contacts are quickly closed, the resultant force with which the moveable contact strikes the fixed contact ensures good electrical conduction between the contacts since impurities, such as dust and dirt, are dislodged. Second, when the contacts are quickly opened, the opportunity for electrical arcing between the fixed contact and the moveable contact is minimized since the contacts are quickly separated, through an arc suppressor, by a distance which is sufficient to prevent such arcing.
The moveable electrical contact is designed not only to strike the fixed contact, when closed, but also to slide across the surface of the fixed contact. This sliding action further aids in ensuring good electrical conductivity between the fixed member and the moveable member.
Despite the above described features, small amounts of debris may, nevertheless, become interposed between the fixed contact and the moveable contact. Under such circumstances, the possibility exists that the fixed contact will weld to the moveable contact thereby preventing the circuit breaker from opening either during an overcurrent condition or during manual operation. Contact welding can also occur due to a mechanical failure of the breaker wherein the force exerted by the moveable contact on the fixed contact is reduced.
The circuit breaker includes a pivoting operating handle, which projects through an opening formed in the breaker housing, for manual operation. The handle may assume one of three positions during normal operation of the circuit breaker. In the ON position, the handle is positioned at one end of its permissible travel. When the operating handle is moved to this position, and the breaker is not tripped, the contacts of the circuit breaker close thereby allowing electrical current to flow from the current source to an associated electrical circuit. At the opposite end of the travel of the handle is the OFF position. When the handle is moved to that position, the contacts of the circuit breaker open, except as described below, thereby preventing current from flowing through the circuit breaker.
A third position is the tripped position which is approximately midway between the ON position and the OFF position. The handle automatically assumes this position whenever the operating mechanism or the magnetic repulsion members have tripped the circuit breaker and opened the contacts. Once the circuit breaker has been tripped, the electrical contacts cannot be reclosed until the operating handle is first moved to the OFF position and then back to the ON position.
As previously described, it is possible for the contacts of the circuit breaker to weld closed thereby preventing the contacts from opening when the circuit breaker is tripped or when the handle is moved to the OFF position. However, the handle may still be manually moved to the OFF position even when the electrical contacts are welded closed and, hence, the electrical circuit is energized. This is because the moveable contacts are mechanically connected to the lever through biasing springs. The handle, therefore, may be moved to the OFF position by overcoming the biasing force of the springs.
This can create a hazard if, for example, a person were to move the handle to the OFF position thereby believing that the electrical contacts are open and that the electrical circuit, connected to the circuit breaker, is therefore, de-energized when in actuality it is not. A person could attempt to manually access energized portions of the circuit and unexpectedly receive an electrical shock. The present invention reduces the risk of such occurrence by providing apparatus which mechanically limits the travel of the operating handle so that it may not be moved to the OFF position when the electrical contacts are closed, such as when they are welded.